(Not Applicable)
This invention relates generally to fuel cells, and more particularly to collector plates for use in fuel cells.
A fuel cell is an electro-chemical device in which chemical energy of fuel and oxygen is directly converted into electrical energy. A basic PEM fuel cell consists of a cathode and anode gas diffusion electrode formed by an electrochemically active catalyst layer that is typically made of platinum. The cathode and anode are separated by a solid, ion conducting, polymer membrane which exists in a hydrated state. The electrodes and membrane form a membrane electrode assembly (MEA) that is enclosed between two electrically conducting, graphite collector plates. An individual fuel cell produces less than one volt at full load, requiring cells to be stacked in series to produce usable voltages. Additional, inactive cells may be added to provide cooling and reactant humidification depending upon the stack""s size and application.
PEM fuel cells operate at relatively low temperatures of approximately 200 degrees Fahrenheit, and typically have efficiencies in excess of 50 percent. They can vary their output quickly to meet shifts in power demand, and are particularly well-suited for applications in which quick startup is required. In a PEM fuel cell, hydrogen-rich fuel is fed through inlet and outlet channels in a bipolar plate. The hydrogen breaks into ions and electrons at a platinum catalyzed membrane. Atmospheric oxygen enters the fuel cell from the opposite side of the membrane. Electricity is generated when the positive hydrogen ions pass through an electrolyte membrane, towards the oxygen, and reacts with the oxygen. After providing power, the electric current joins the hydrogen ions and oxygen to produce water. Water product can be trapped within the gas diffusion layer, and efficient removal is desirable.
Reactants and reaction products are transported to and from the fuel cell membrane assembly through passages formed in the collector plates. These passages, typically formed as channels, can extend continuously from inlet to outlet, referred to as open channel, and provide gases to the gas diffusion layers laterally. Alternatively, the channels can be formed discontinuously, and positioned adjacent each other, so that gas in an inlet channel is forced through the gas diffusion layer and exhausts into an outlet channel. In this discontinuous arrangement, the inlet and outlet channels can be arranged in an interdigitated relationship or in spiral series.
As gases flow in the channels, the reactants are transferred into the gas diffusion layer, and thus the concentration of reactant in the flow channels is reduced along its length of travel. This reduction in concentration can result in non-uniform reaction across the fuel cell active area.
Thus, it would solve problems of the prior art to provide collector plates having various channel designs to improve water removal and/or improve the uniformity of the flow of reactants in the gas diffusion layers.
It is an object of the invention to provide fluid passage arrangements that improve the uniformity of reactant distribution across the fuel cell.
It is a further object of the invention to provide for more uniform distribution of reactants in a fuel cell using a variety of channel geometries.
It is yet another object of the invention to improve the removal of water product from fuel cells through the collector plate channel construction.
These and other objects of the invention are provided by fuel cell collector plates having uniquely arranged channel constructions. A collector plate for use in a fuel cell system can include a generally planar collector plate body formed at least partially of conductive material and defining two opposed, substantially parallel planar surfaces surrounded along a periphery by a plate edge, in which at least one of said planar surfaces has at least one inlet channel extending from an inlet port, and at least one outlet channel extending from an outlet port, for the flow of fluids through said collector plate, with the inlet channel and said outlet channel being spaced apart on said at least one planar surface. According to an aspect of the invention, the outlet channel has an outlet channel volume and said inlet channel has an inlet channel volume; and the outlet channel volume is less than said inlet channel volume, whereby the rate at which the fluids flow to the outlet channel is increased, causing improved fluid removal.
According to another aspect of the invention, at least a portion of the cross-sectional area of the outlet channel is less than at least a portion of the cross-sectional area of the inlet channel. Various channel geometries can be provided. For example, the outlet channel volume can have a cross-sectional area having an outlet channel width, and said inlet channel volume can has a cross-sectional area having an inlet channel width, in which the outlet channel width is less than the inlet channel width. Alternatively, the outlet channel depth can be less than said inlet channel depth. Also, both the outlet channel width can be less than said inlet channel width, and the outlet channel depth less than said inlet channel depth.
According to another embodiment of the invention, the length of the outlet channel can be less than the length of the inlet channel. This geometry can be achieved, for example, by a spiral arrangement in which the inlet channel extends in a spiral adjacent to a spiral outlet channel inset at a smaller radius than the inlet channel spiral. In other words, the inlet channel and the outlet channel are arranged as spirals, said outlet channel spiral extending along a radially interior side of said inlet channel spiral, whereby the overall length of said outlet channel spiral is less than the overall length of said inlet channel spiral.
The inlet and outlet channels can also be interdigitated.
According to another aspect of the invention, collector plate channels can be constructed to include at least one inlet channel extending from an inlet port to an inlet terminus and at least one outlet channel extending from an outlet port to an outlet terminus for the flow of fluids through said collector plate, said inlet and outlet channels being spaced apart and adjacent to each other on said at least one planar surface to define a transfer zone between said inlet terminus and said outlet terminus, in which said portions of said inlet and outlet channel portions in said transfer zone define a land having a land width. The land width decreases as the fluids flow in the direction of the inlet terminus.
The decreasing land width can be achieved by a number of channel geometries. The inlet channel can increase in width towards its terminus. The outlet channel can increase in width away from its terminus. Alternatively, both can increase, resulting in a closer positioning of the inlet channel and the outlet channel in the direction of flow of the fluids.
This reduced land width permits greater mass flow rates as the concentration of reactants is decreasing. This advantage can be coupled with improved water removal by decreasing the outlet channel volume as well, by reducing the depth of the outlet channel relative to the inlet channel and/or reducing the outlet channel length relative to the inlet channel length.
Preferably, the decreasing land width in the direction of flow is utilized in an interdigitated channel configuration. The interdigitated arrangement can be linear or circular.
According to another aspect of the invention, an open channel arrangement can be constructed to reduce the diffusion path as the concentration of reactant in the flow is depleted. In such a continuous flow line, the depth of the channel can be reduced in the direction of flow so that the diffusion path is correspondingly reduced. As reactant is transferred into the diffusion layer during fuel cell operation, the concentration of reactant in the remaining flow is reduced. However, according to the invention, the depth of the channel decreases, so that the relatively lower concentration of reactant is more readily transferred into the diffusion layer. The width of the channel can also be reduced.
Thus, a number of collector plate channel arrangements can contribute to more effective water removal and more uniform distribution of reactant into the gas diffusion layer.